Plasma arc torches are widely used in the processing (e.g., cutting and marking) of metallic materials. A plasma arch torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is constricted to produce a jet of plasma gas with high temperature and high momentum. The gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
In plasma arc cutting or marking a metallic workpiece, a pilot arc is typically first generated between the electrode (cathode) and the nozzle (anode). Controlled gas flow pushes the pilot arc through the nozzle and out the exit orifice. The pilot arc extends beyond the nozzle until the electrical resistance between the electrode and the workpiece is reduced enough to allow transfer of arc attachment from the nozzle to the workpiece. Cutting and marking are done with the torch operating in this transferred plasma arc mode, characterized by the flow of electrons and conductive ionized gas from the electrode to the workpiece.
In a plasma arc torch using a reactive plasma gas, it is common to use a copper electrode containing an insert made of a material capable of thermionic emission, which can be in the form of an insert. The insert is press fit into the bottom end of the electrode so that an end face of the insert, which defines an emission surface, is exposed. The insert is typically made of either hafnium or zirconium and is cylindrically shaped.